Evaporation of gasoline from motor vehicle fuel systems is a major potential source of hydrocarbon air pollution. The automotive industry is challenged to design engine components and systems to contain, as much as possible, the almost one billion gallons of gasoline evaporated from fuel systems each year in the United States alone. Such emissions can be controlled by canister systems that employ activated carbon to adsorb and hold the vapor that evaporates.
A typical canister system employed in a state of the art auto emission control system is shown in FIG. 1. An active purge canister 2 is connected to the fuel tank 1. The active purge canister 2 has a canister vent 3 and a purge line to the engine intake manifold 4. Some basic auto emission control system canisters are disclosed in U.S. Pat. Nos. 5,456,236; 5,456,237; 5,460,136; and 5,477,836.
These canisters and canister systems work to adsorb hydrocarbons released from the fuel system. Under certain modes of engine operation, the adsorbed hydrocarbon vapor is periodically removed from the carbon to form “regenerated carbon” by actively drawing air through the canister (“active purge”) and burning the desorbed vapor in the engine. This regenerated carbon is then ready to adsorb additional vapor.
Typically, this “active purge” takes place while the engine is running and the engine manifold creates a vacuum which is used to draw air through the carbon canister. In order to completely purge a carbon canister back to its designed free capacity and to meet allowable emissions level, a significant volume of purge air is required (typically 100 to 400 times the volume of the canister).
Another means of carbon regeneration takes place through “passive purge” or “natural backpurge.” The natural backpurge takes place throughout the day as the ambient air temperature drops. Typically, the warmest air temperatures are during the late afternoon and the coolest air temperatures are during the early morning hours. This natural temperature fluctuation is termed diurnal temperature fluctuation. As the ambient temperature increases, the temperature of the vehicle's fuel tank and its contents also increases. This temperature increase causes the air and vapors to expand and become less dense, and the increase also causes a rise in fuel vapor pressure and the evolution of fuel vapor to maintain vapor-liquid equilibrium. The net effects of these processes include: (1) a rise in gasoline vapor concentration within the tank vapor space, and (2) the venting of gasoline vapors and air from the fuel tank to maintain pressure equalization with an open line to atmosphere (or pressurization of the fuel tank if the tank is sealed from the atmosphere).
When the ambient temperature declines, the temperature of the fuel tank and the tank's contents also declines, and the opposite effect occurs. The air and gasoline vapors contract and increase in density. The vapor pressure of the fuel declines and causes fuel vapors in the ullage to condense into the liquid as the equilibrium vapor concentration declines. The net effect of these processes is the drawing of air into the fuel tank. When a carbon canister is placed in the tank vent, the air drawn into the tank during periods of ambient temperature drops is able to provide limited regeneration of the canister.
A typical 60 liter fuel tank, 40% filled with 7 psi Reid Vapor Pressure (RVP) gasoline, will vent approximately 35 grams of hydrocarbon vapor when heated from 65° F. to 105° F. A typical commercialized carbon canister will adsorb over 99% of these vapors for one to three days or more, depending upon need and design. When this same tank system is cooled from 105° F. to 65° F., approximately 5 to 7 grams of hydrocarbons will desorb and vent back to the fuel tank. Over a 24-hour cycle, the net carbon canister load is 28 to 30 grams.
Automotive manufacturers and systems designers are seeking methods to significantly reduce the volume of active engine purge required to regenerate the carbon canister and provide both the necessary level of adsorption capacity and meet vented emissions targets. Automotive manufacturers and system designers are also seeking methods to decrease the mass of gasoline vapors that must be delivered to the engine during purge. Automotive manufacturers are also seeking methods to reduce the average concentration of fuel vapors in the purge air by reducing the hydrocarbon concentration; the volume of purge air per unit time drawn through the canister to the engine may be increased with minimal impact on engine controls. For the system described above (a 60 liter fuel tank, 40% filled with 7 psi RVP fuel, undergoing a 24-hour diurnal cycle of 65° F. to 105° F. to 65° F.), the fuel tank will vent outward approximately 15.3 liters of air plus approximately 9.7 liters of gasoline vapors. A typical carbon canister applied to this system, designed to meet U.S. Federal emission standards, will be filled with an 11 or a 15 g/100 mL Butane Working Capacity (BWC) carbon to a volume of approximately 2 liters. The volume of Active Engine Purge required to suitably purge this canister after three days of loading will be 200 to 800 liters, or 4.4 to 17 times the total volume of air comprised in the load stream.
New engine technologies (such as gasoline direct injection) and hybrid electric arrangements are reducing the availability of purge air or ability to handle large hydrocarbons loads for carbon canister regeneration. Gasoline engines on hybrids only run intermittently, which reduces purge time. Gasoline direct injection engines operate with intake manifold vacuum levels much closer to atmospheric pressure than traditional internal combustion engines, which reduce the driving force for developing purge flow through the canister. Automakers have established targets of 10 to 75 bed volumes of purge (or 20 liters to 150 liters of purge air for the example discussed above) air to accommodate these new technologies. These targets are based on either limitations for developing purge volumes or limitations on the mass of hydrocarbons the engine can tolerate, yet maintain drivability and exhaust targets, based on current hydrocarbon concentration in the purge.
Therefore, new technologies and innovations are required to allow these new engine technologies to meet environmental emission control standards. Automakers have also set these targets, not simply because of a need to reduce the volume of air purge, but also because new hybrid technologies and new environmental exhaust standards are reducing the amount of time available to purge. With current state of the art canister systems, a large purge volume equates to a high gasoline vapor purge load to the engine. Exhaust catalysts and engine controls cannot adequately fully oxidize the purged hydrocarbons in the allotted time and meet exhaust requirements. The ability to supply high volumes of purge air to the canister is not always the primary goal; the primary goal is often a reduction in the mass of hydrocarbons sent to the engine during purge. Thus, an alternative need is to reduce the mass of hydrocarbons in the canister that must be actively purged by the engine to fully regenerate the canister.
The present disclosure meets such innovative needs. One purpose of the present disclosure is to provide a cost-effective technology to meet these new purge targets established by the automakers to successfully bring new engine technologies to the consumer. Further purpose is to reduce the amount of gasoline vapors that will be purged from the canister/canister system and delivered to the engine with the purge air.